1. Field of the Invention
The invention relates to a filling material for percolating filters for the treatment of fluids, especially for biological sewage purification. The invention relates, furthermore, to a process for the production of filling material serving for the treatment of fluids.
2. Description of Related Art
The percolating filters under discussion here are used predominantly in sewage technology for biological sewage treatment. However, they can also be used for other treatment of liquids and gases, especially waste gases, such treatment also preferably taking place biologically. The percolating filters serve as carriers for the bacteria necessary for biological treatment, particularly of the so-called biological lawn. The efficiency of a sewage-purification plant equipped with filling material of this type is determined by a plurality of factors. On the one hand the filling material is to offer as large a specific growth surface as possible, and on the other hand as good a hold as possible of the bacteria on the filling material is to be guaranteed. Finally, the fluid to be treated, a liquid or a gas, is to be capable of flowing round or wetting the biological lawn on the filling material without any obstruction.
EP-B 0 274 633 discloses a filling material of the relevant generic type, strips of which are connected to one another by means of a holding web directed transversely relative to the longitudinal direction of these. The strips have identical cross-sections over their length. They are provided with curved-up portions on opposite edge regions. In the middle, the strips have a flat shape. It has proved to be disadvantageous that, as a result of the weight load of the biological lawn settling on the strips, in course of time the strips are pulled flat and consequently gradually stick together. The fluid therefore can no longer flow through between the biological lawn of adjacent strips. The effectiveness of a sewage-treatment plant provided with such filling material is thereby reduced.
To avoid the abovementioned disadvantage, a filling material; on the strips of which a single strand is arranged approximately centrally, is known from EP-B 0 332 907. The strips likewise have identical cross-sections over their length. To increase the strip density, the strips have to be arranged one above the other in a plurality of layers. It has been shown that the fluid to be treated is distributed only unevenly over the different layers of the strips. The effective strip density is consequently restricted essentially to the upper layer.
Furthermore, it has proved complicated to form filling material with a plurality of layers of strips arranged one above the other.